Accuracy is the degree of conformity of a measured or calculated quantity to its true value. Precision or repeatability is the degree to which a number of measurements or calculations show the same or similar results. The results of a measurement can be accurate but not precise, precise but not accurate, neither, or both. The measurement is valid if it is both accurate and precise.
To determine if a measuring device makes valid measurements, a standard with a known dimension can be used. The measuring device is used to measure the standard and the resulting measurement compared to the known dimension of the standard. If the measurement produced by the measurement device is substantially equivalent to the known dimension and the measurement is repeatable, it can be assumed that the measurement device is calibrated to make valid measurements of that particular dimension.
This calibration allows the accuracy/precision of the measuring device to be ascertained over its lifetime and corrective action taken when the accuracy/precision falls below a predetermined value. These measuring devices may include tape measures, calipers, micrometers, protractors, or other devices.
Throughout the drawings, identical reference numbers designate similar, but not necessarily identical, elements.